Polymer products, particularly acrylic products, having a granite appearance and methods for their production are known in the art. Such products have found particular application in the spa and bathware industries. Success of acrylic sheet products having a granite appearance, such as Lucite® XL Natural Series Granite manufactured by the assignee of the inventors, in the spa and bath marketplaces has recently created a particular customer need for granite-like acrylic sheet products with variegated appearance. This is especially valuable in the marketplace if the desired variegated appearance is apparent after the acrylic sheet has been thermoformed into its final shape as a spa, bathtub or vanity bowl.
In general, known methods for the production of acrylic compositions to form acrylic sheet products having a granite appearance comprise the addition of various inorganic or organic particles or fillers to an acrylic solution or syrup. For example, U.S. Pat. Nos. 4,159,301 and 4,085,246 disclose a simulated granite material comprising a matrix of acrylic polymer and various opaque and translucent particles having specific optical densities. U.S. Pat. No. 5,304,592 discloses a simulated mineral article comprising a plastic material suspended within a thermoplastic matrix. The plastic material comprises both a thermoplastic and thermoset plastic and the matrix consists essentially of a thermoplastic material. U.S. Pat. No. 5,043,077 discloses a granite-like artificial stone comprising a matrix of radical-polymerizable monomers (containing methacrylate and an aromatic vinyl compound) and an inorganic filler, where the filler comprises radical-polymerizable monomers which may be the same or different than those used in the matrix. U.S. Pat. No. 4,959,401 discloses a composition suitable for making synthetic rock comprising an organic portion comprising one or more polymers and an inorganic filler portion comprising an oxide pigment, kaolin and a binder. However, each of these compositions have demonstrated difficulties in processing and later thermoforming.
In an attempt to advantageously affect the processing and thermoforming characteristics of the compositions suitable for forming granite-like products, U.S. Pat. Nos. 5,242,968, 5,415,931 and 5,530,041 provide an acrylic product having a matrix of polymethyl methacrylate containing preformed particles of polymethyl methacrylate. The preformed particles are preferably scrap material and comprise greater than 90% of polymethyl methacrylate and 1% of crosslinker, the remainder of the particles being comonomer.
The use of a polymeric material as the particulate or filler component of such compositions has been found to be preferred over the various fillers previously used. Such particles provide sufficient opacity to the composition for hiding and decorative purposes. However, care must be taken in formulating such compositions since the polymeric particles will swell due to the absorption of the matrix monomer to a volume that may be several times greater than their initial volume. The swelling of the particles will increase the viscosity of the composition and prevent the particles from settling.
Such compositions have been used in practice only with difficulty, as the size and swelling of the particles have not been heretofore optimized. In order to produce a material which is easily prepared, aesthetically pleasing and particularly suitable for further processing, the shape, size and swelling rate of the particles must be controlled and optimized. In conventional methods for preparing granite-appearing acrylic materials, such as that disclosed in the U.S. Pat. Nos. 5,242,968, 5,415,931 and 5,530,041 patents, the particles are often made from cell cast, continuous cast or extruded sheet material. These particles are often difficult to use as they either swell to an extent or at a rate which does not provide optimum results.
If the particles swell too much, the composition may absorb nearly all of the liquid or matrix phase, making its viscosity so great as to be unworkable. Alternatively, the particles may dissolve, losing their character altogether. If the particles do not swell to a sufficient degree, the mixture may not increase in viscosity sufficiently, allowing the particles to settle, thus detracting from the granite aesthetic. On the other hand, if the particles swell too slowly, the manufacturing process must be designed to accommodate a dynamic mixture which is constantly changing in viscosity or sufficient processing time must be allowed to allow the composition to reach equilibrium.
Accordingly, U.S. patent application Ser. No. 08/544,375 now U.S. Pat. No. 5,880,207 provides for acrylic compositions suitable for processing into a product having a granite appearance which are formulated to allow the processor to optimize the extent of particle swelling, the rate of particle swelling and the thermoforming properties of the final product, and a process for preparing such an acrylic composition.
However, to achieve the desired variegated appearance or effect, the particles need to have a certain morphology.
In the U.S. Pat. Nos. 5,242,968, 5,415,931 and 5,530,041 patents, the particles are not described as having any particular morphology, but only of a certain size which is predominantly from about 0.1 mm to about 2.0 mm. Optical microscopy measurements of the particles described in U.S. Pat. No. 5,880,207 reveal a predominantly spherical/cubical morphology. (Thickness, width and length are similar within each particle).
The conventional method of producing the predominantly spherical particles is grinding with an impact mill unit such as a Pallmann Pulverizer. Particle size distribution may be controlled by changing screen or sieve sizes. For example, a 30-60 mesh produces particles in the range of 0.2 to 0.6 mm in diameter. Formulations containing the spherical/cubical particles, however, tend to produce acrylic sheets with these particles positioning themselves more on the bottom side of the sheet when the matrix containing the particles is pumped onto a belt polymerizer.
The surface area of the spherical particle is such that they randomly orient themselves within the mixture of smaller spherical particles in the matrix, which results in the larger spherical particle sinking to the bottom of the matrix. Such settling of the larger spherical particles to the bottom also has the undesired effect of reducing sheet extension during thermoforming. A further limitation of the inclusion of spherical particles in the matrix is that they cannot be used in thin gauges of sheet, down to 0.06 inch.
It is an object of the present invention to provide a process for producing a new particle morphology, which will allow the particles to stay at the top of the acrylic matrix on the belt polymerizer and thereby produce a sheet with the desired variegated appearance instead of the normal “granite” look that is more homogeneous.